class-wc-product-grouped.php

<?php
// replace the foreach loop on line 132 with this version:
foreach ( $this->get_children() as $child_id ) {
	
	if ( ! is_int( $child_id ) ) {
		continue;
	}
	
	$sale_price = get_post_meta( $child_id, '_sale_price', true );
	if ( $sale_price !== "" && $sale_price >= 0 ) {
		$is_on_sale = true;
	}
	
}